Reference

30-Minute SQL Mock

Interview Mock Timer

Use this timer to simulate interview pressure. Prioritize correctness, then explain tradeoffs.

Duration

20:00

Use this lesson to simulate a realistic SQL interview with strict timing and evaluator-style scoring.

Round Plan (30 Minutes) #

0-5 min: clarify assumptions and output grain
5-18 min: write first-correct query
18-25 min: add validation and edge-case handling
25-30 min: explain tradeoffs and final answer

Mock Prompt #

For each month, return completed-order revenue, purchasing customers, and month-over-month revenue growth. Revenue is SUM(order_total) over completed orders (order grain, so do not sum line items); growth is the month-over-month percent change.

Required output columns (use these exact aliases):

month_start
revenue
purchasing_customers
mom_revenue_growth

Loading SQL editor...

Validation Queries #

sql

-- Revenue baseline
SELECT
  DATE_TRUNC('month', order_date) AS month_start,
  ROUND(SUM(order_total), 2) AS revenue
FROM ecom_orders
WHERE order_status = 'completed'
GROUP BY DATE_TRUNC('month', order_date)
ORDER BY month_start;

-- Grain check (one row per month)
WITH monthly AS (
  SELECT DATE_TRUNC('month', order_date) AS month_start
  FROM ecom_orders
  WHERE order_status = 'completed'
  GROUP BY DATE_TRUNC('month', order_date)
)
SELECT month_start, COUNT(*) AS row_count
FROM monthly
GROUP BY month_start
HAVING COUNT(*) > 1;

Evaluator Scorecard #

Correctness (40%): SQL returns correct values at correct grain
Validation (25%): evidence query confirms logic
Communication (20%): assumptions/tradeoffs are explicit
Time management (15%): solution completed within round

Common Fails in Timed Rounds #

coding before clarifying definitions
no validation query
incorrect denominator logic for growth/ratios
unfinished answer due over-optimization

Tip

Under time pressure, first-correct plus validation beats clever but unverified SQL.